Bipolar transistors and MOSFET transistors represent two different families of semiconductor devices which each have distinct advantages over the other. The operation and structure of these semiconductor families have traditionally differed, and thus have experienced different development routes to exploit the individual advantages. For example, bipolar transistors are well suited for use in high power, high speed, digital and analog applications. On the other hand, MOSFET transistor devices, including PMOS and NMOS transistors which form CMOS devices, are well suited for low power and high packing density applications.
The electrical operation of a bipolar transistor involves both minority and majority carriers, and has traditionally been fabricated in a manner different from CMOS transistor circuits which depend upon current flow involving majority carriers. In addition, because the operating characteristics of bipolar transistors depend upon the lateral geometry, as well as the vertical geometry of semiconductor regions, the fabrication thereof has traditionally taken a different route then that of CMOS transistors, which are lateral surface-operating devices.
With the current trend toward a large scale integration of semiconductor circuits, it has become advantageous to integrate bipolar circuits and MOSFET circuits into the same chip. In this manner, many MOSFET circuits can be arranged in a small wafer area and utilized to perform an electrical function, while the current driving capabilities of bipolar transistors can be used as the drivers for such MOSFET circuits. Many other applications exist in which the advantages of both MOSFET and bipolar devices can be combined into a single integrated circuit chip to provide an overall improved performance.
The initial integration of bipolar circuits and MOSFET circuits involved conventional process steps to form the MOSFET devices, as well as conventional steps to form the bipolar devices. Very few processing steps were shared between the fabrication of each such device type, and thus the overall process was complex, lengthy, costly and susceptible to low production yields. Because of the increased importance of integrating bipolar and CMOS devices, many attempts have been made to develop processes whereby various structures of both types of devices can be fabricated simultaneously such that the total number of processing steps are minimized, and the technologies are brought into correspondence, without compromising the performance or advantages inherent in each type of device.
From the foregoing it can be appreciated that a need exists for an improved bipolar transistor device which can be fabricated with process steps highly compatible with those of MOSFET devices. There is an associated need for a bipolar device, and method of fabrication thereof, which results in a transistor occupying less wafer area and improved performance.